In a CDMA mobile communication system, the power control will directly affect the system communication performance, which mainly refers to communication capacity and call drop rate.
The method of controlling the forward power in a CDMA mobile communication system usually includes open loop, closed loop, and outer loop control, which are used to control the power of transmitting signals form a base station to a terminal, wherein in the closed loop control, a control range of the forward transmission power, i.e. the control threshold Ptx_lower and Ptx_upper, needs to be set, and the actual forward transmission power is constrained within the range of these two power thresholds. Ptx_lower is the lower bound of the forward transmission power, if it is set too low, call drop will easily happen, especially in fast fading areas; and if it is set too high, the system capacity will be affected. Ptx_upper is the upper bound of the forward transmission power; if it is set too low, voice quality can not be assured in places of bad signal quality, and call drop happens easily at the same time; and if it is set too high, the system capacity will also be affected, resulting in the overload of the forward transmission power easily. Therefore, determining suitable upper bound and lower bound of the forward transmission power is important for promoting the performance of CDMA mobile communication system. Usually, in a system (or in a sector), Ptx_lower and Ptx_upper are fixed, which are experience values under theoretical instructions. Due to the particular complexity of the mobile communication, such as the greatly variant traffics in different time periods and the different transmission modes, the forward transmission power is actually fluctuant. The fixed Ptx_lower and Ptx_upper can not trace such variations in real-time, and can not adapt to the changes of the dynamically varying communication environment and the fluctuation of the call capacity. Generally, in different time periods, due to an increase of the traffic, signal distortion will occur if the sector forward transmission power is overloaded, which will greatly affect the performance of the communication system. In the case of the overloaded sector power, how to ensure the whole system can quit the overload state as soon as possible and ensure the bad effects to the system can be reduced to the minimum during the course of quitting the overload state is essential to the power control.
Currently, there are some methods for dynamically regulating the forward power in a CDMA mobile communication system, such as the method of channel adaptive power control. In this method, by using a group of different TPC algorithms (TPCAs) of random size, the transmission power can be controlled in a channel adaptive mode. Each TPCA should adapt well to a particular type of channel. In the method, different algorithms and rules for determining whether an algorithm is good should be set. However, using different algorithms to track the channel can not ensure the feature of real-time.
Furthermore, overload control usually adopts a method of multi-level control. During the implementation, many levels, such as forbidding a new call, forbidding additional channel assignment, forbidding soft handover and forbidding terminal power increase, etc. are to be defined. Under such overload control, transmission power of all terminals are finally controlled, i.e. during the course of overload, the forward transmission power of the terminal should not exceed the transmission power at the moment of entering the overload state (which is analogous to reduce the upper threshold of the forward transmission power). In such way, since the forward transmission power of a good link is originally low, if its transmission power increase is restrained, it may result in a call drop, and also because the transmission power is very small, its contribution to the reduction of overload after the call drop is not significant.
Based on the discussion above, how to optimize the forward power control effect in a CDMA mobile communication system and better control the sector forward transmission power overload of the system is an important technical problem needing to be solved by the skilled in the art.